Solder platings on mechanical parts or like substrates impart high corrosion resistance and good solderability to the substrate. Solder platings therefore have been widely applied to automotive parts or the like and in recent years to electronic equipment, precision mechanical equipment, printed circuit boards or the like.
Conventional baths for electrolessly solder plating include, for example, a borofluoride bath consisting essentially of borofluoric acid, stannous borofluoride and lead borofluoride, a chloride bath consisting essentially of stannous chloride, lead chloride and hydrochloric acid and containing thiourea, etc.
However, these baths have the problems resulting from the composition which include those of (a) being likely to corrode the substrate; (b) leaving waste water containing harmful ingredients; (c) deteriorating the work environment; and (d) evolving a halogen gas or the like from the plated substrate.
Stated more specifically, the borofluoric acid or hydrochloric acid present in the conventional electrolessly solder plating bath unavoidably errodes part of substrates of copper, copper alloy or the like. Accurately patterned components such as printed circuit boards, copper foil circuits or the like tend to be adversely affected as by variations of electroconductivity caused due to breaking of wire or varied wire width. To meet the requirement of semiconductor parts in computers being essentially free of halogen ions, a procedure is needed for washing plated substrates to remove the halogen ions. If the removal of halogen ions is incomplete, the plated substrate becomes susceptible to change of color or corrosion.